In steam plants using saturated steam, condensation due to energy losses results in a liquid phase in steam. Condensation begins as sub-micron size droplets on nucleation centers. Additional droplets may also be present at the output of the steam generator due to inefficiencies in the equipment intended to remove them. The droplets increase in size as the steam travels to the turbine, energy is lost, and condensation occurs. Droplets may also combine creating larger droplets. Droplets with a distribution of sizes and number densities will exist at any point in the wet steam and the distributions will change from point to point.
The resulting wet steam can cause damage to components of the steam system. For example, droplets entering a low pressure (LP) turbine can cause erosion, eventually resulting in the failure of turbine blades. Piping erosion caused by the droplets, in combination with corrosion, could lead to failure of the piping.
In view of the above, it is desirable to measure the liquid phase of steam to thus provide indications of premature failure in steam system components. Moreover, measurement of wet steam concentrations and velocities provides a viable estimate of turbine efficiency.
Optical instrumentation has been used in the past to measure particle size and velocity. Generally, the physical basis of optical detection in particle characterization is the scattering and absorption of light by the individual particles. Presently known devices for measuring particle size and velocity can be categorized as being based on single particle scattering or multiple particle scattering. In single particle scattering, the particle size, and/or velocity is determined for individual particles traversing a finite probe volume. Information about specific distributions of particles is obtained with statistical techniques from the individual particle events.
In multiple particle scattering, the scattered light from a large number of particles is measured to determine particle size and distribution. Measurements of scattering from multiple particles either use the extinction of light through a large probe volume or the angular dependency of Mie scattering to characterize the particles. An a priori assumption about the number-size density distribution function is usually necessary in multiple particle techniques.
Optical measurement of the liquid phase in steam was reported in a paper entitled "Determination of Droplet Sizes and Wetness Fraction in Two-Phase-Flows Using a Light-Scattering Technique (High Pressure Live Steam of Nuclear Power Plants, Low Pressure Steam Turbines, Cooling Tower Plumes) by A. Ederhof and G. Dibelius, IMechE 1976 (Conference Publication--Sixth Thermodynamics and Fluid Mechanics Convention, Durham, N.C.). For the light scattering probe described therein, a laser beam was focused into the steam flow of a live steam line. The liquid phase of the wet steam is thus characterized by measuring the intensity of light scattered at an angle of 90.degree. from single particles interacting with a narrow collimated laser beam.
A multiple particle technique is described in "Water Droplet Size Measurements in an Experimental Steam Turbine Using an Optical Fiber Droplet Sizer" by K. Tatsuno and S. Nagao, Journal of Heat Transfer, Vol. 108/941 (November 1986). The optical droplet "sizer" described therein uses the forward scattering method, in which the angular sensitivity of Mie scattering was used to characterize multiple particles within a defined probe volume.
Another multiple particle technique is described in "An Optical Instrument for Measuring the Wetness Fraction and Droplet Size of Wet Steam Flows in LP Turbines" by P. T. Walters and P. C. Skingley, I Mech E 1979 (Conference Publication--Design Conference on Steam Turbines for the 1980's, London, United Kingdom). In this apparatus, multiple steam particles are characterized with a multi-wavelength extinction. The droplet size and wetness fraction of flows in LP steam turbines is obtained by measuring the optical transmission of a sample flowing through a slot in a probe body. Data generated thereby is interpreted from light scattering theory to give values for size and distribution.
In general, the instrumentation devices described above are sensitive to unwanted scattering outside of the probe volume. Each device isolated the optical probe volume from extraneous scattering by containing the probe volume within a radial arm extending into the steam environment. The measured scatter signal was transmitted using various optical techniques, such as fiber optics, through the radial arm to photodetectors located outside the steam pipe. Measurements were made at different radial locations within the steam environment by moving the radial arm. Measured particle sizes ranged from 0.1 .mu.m to 500 .mu.m under a variety of conditions. Most measured particle sizes in LP turbines were in the 0.8 .mu.m range.
A problem inherent to the type of instrumentation described above is that they are invasive. In addition to the problem of placing a radial arm through the wall of a steam pipe, problems are encountered in determining the effect of the radial arm on the steam flow and therefore on the measurement itself. As a result, a large variability in steam particle sizes have thus been measured under similar conditions.
Another problem is related to the way in which the scattered light is analyzed. The aforementioned instrumentation are designed to measure phase shift of the scattered light at different radial locations as a particle traverses the probe volume. For example, see "Simultaneous Measurement of Size, Concentration and Velocity of Spherical Particles by a Laser Doppler Method" by M. Saffman et al. (paper presented at The Second International Symposium on Applications of Laser Anemometry to Fluid Mechanics, Jul. 2-4, 1984, Lisbon). These instruments require several independent views of the probe volume and thus dictate either an arm extending into the steam environment or the use of multiple ports in the steam pipe or turbine housing. However, when making measurements of process steam, it is desirable to minimize the number of access ports to the environment. This is necessary not only from a logistics standpoint with the difficulty in the initial set-up and alignment of multiple optical ports, but also from a maintenance standpoint in keeping the optics aligned in the presence of large vibration sources and thermal stresses.